1. Field of the Invention
This invention relates to a metering valve and, more particularly, to a valve for providing measurable fine flow control by nonrotational longitudinal movement of a valve stem sealed in a bellows and connected by a coupling to an actuator that is rotatably mounted for axial movement in a bonnet of the valve.
2. Description of the Prior Art
Metering bellows valves for precisely controlling the flow of toxic, corrosive, or expensive liquids or gases through a piping system are well known in the art. U.S. Pat. No. 3,428,291 is an example of a bellows metering valve having a nonrotating stem that is incrementally movable to assume accurate sealing of a valve member relative to a valve seat. In view of the fact that a bellows surrounds part of the stem and is connected to the upper end of the stem, the stem must be prevented from rotating and exerting torsional loads on the bellows when the actuator is threadedly rotated within a bonnet.
As disclosed in the above-identified patent, rotation of the stem upon rotation of the actuator is prevented by guiding the stem at its lower end by a washer and at its upper end by a bore in the bonnet where rotation of the stem is prevented by interaction of a flat on the washer with a corresponding flat on the stem. Fine adjustments of the flow rate through the valve are obtained by a differential thread mechanism that includes relatively coarse threads on the exterior of the bonnet and finer threads on the end of the valve stem with an actuator threadedly engaging with a first threaded portion the coarse threads on the bonnet and with a second threaded portion the fine threads on the valve stem. As the actuator is rotated relative to the bonnet and the stem, the stem advances a net amount equal to the difference in the pitch between the bonnet and the stem threads to provide accurate setting for the flow through the valve.
U.S. Pat. Nos. 3,278,156 and 3,428,291 disclose bellows metering valves in which the actuator is threadedly retained in the bonnet and includes a socket end portion having a conical recess for receiving a dome-shaped end portion of the valve stem. The end of the stem is retained in the conical recess by a pin extending through the conical recess and engaging a radial groove on the end of the stem within the recess. Longitudinal rotational advancement of the actuator generates non-rotational longitudinal movement of the valve stem as a result of the connection of the valve stem to the actuator.
U.S. Pat. Nos. 3,679,169 and 3,751,053 disclose high pressure needle valves in which a valve needle is non-rotatably retained in a valve body. An actuating stem is longitudinally rotationally mounted in the valve body and includes a socket-like end portion that receives an enlarged spherical end portion of the valve needle. A positive contact of the spherical end portion is maintained in the socket of the actuating stem so that rotational, longitudinal movement of the actuating stem is converted to non-rotational movement of the valve needle. U.S. Pat. No. 3,356,335 is another example of a metering valve that provides interconnecting means between a handle or actuator with the stem whereby rotational movement of the handle generates longitudinal movement of the stem without rotation of the stem.
While it has been suggested to provide metering valves with non-rotational longitudinally movable valve stems, some of the known mechanisms for connecting the rotational actuator to the stem include direct contact of the actuator with the stem. This means of connection, while effective to prevent rotation of the valve stem, is subject to wear at the point of contact of the stem with the actuator. The wear at this point between the actuator and the stem is undesirable, particularly in a metering valve where precise flow control is required. Consequently, the contacting parts must be frequently inspected to insure positive response of the valve stem to rotation of the actuator. The other known arrangements which provide for interconnecting means between the actuator and the stem in which the stem and the actuator do not contact each other require complex machining to assure that the desired accuracy of the flow control is maintained.
Therefore there is need in a metering valve, and particularly in a bellows metering valve, for an arrangement that connects the actuator to the valve stem in a manner that does not require complex machining and prevents wear of the connected parts where solid contact is maintained between the connection of the actuator to the valve stem in order to obtain precise, repeatable flow settings.